kiji-rest: REST-64. Fix exception handling.

Fix the exception handling by removing the default DropWizard exception handler
and replace it with a custom one that returns JSON along with logging to the
console through the Log4j logger. Remove any other exception mappers as they all
should look the same.

Reviewer: shashir
commit: https://github.com/kijiproject/kiji-rest/commit/838e8898ac4c378b0e568fdd809a9a7d63127a1e